ORIGINAL: downunder
ORIGINAL: DarZeelon
...I think it was detonation, which ceased with a slight mixture change.
...he made no mention of richening the mixture. Personally I doubt it was detonation in the first place because detonation will normally show itself at full load at the point of maximum torque. The engine in question was a .76 driving an 11x7 prop so should have been running well above max torque revs. Even at peak torque you won't get it unless you're pushing the engine to its limits with the highest nitro, a totally incorrect plug or badly cowled and overheating.
Brian,
I did not imply he actually turned the needle out to change the mixture strength.
It is enough for the ambient temperature, or relative humidity, to change a little, to create enough of a change in the mixture strength, to preclude detonation.
Just for the record,
XJet's friend is using an 11x8 on this GMS (see post #1), so despite this prop also being smallish for the engine, it was closer to the torque peak than you wrote..
Detonation will more likely take place at very high cylinder pressures, associated with maximum torque.
But swirl and atomization could make the mixture more sensitive higher up the rev range.
I have lately seen an engine that detonated just short of max RPM, during acceleration.
It was enough for the prop driver to eat into the hub and it ceased completely, after the
low-speed needle was opened less than 1/8th of a turn.